Moulds made of porous material for the production of ceramic objects comprise one or more forming cavities, each delimited by a surface designed to form the outer surface of the ceramic object and connected to a network of drainage channels and a system for filling the forming cavity with the ceramic mixture and emptying the cavity. Special drainage manifolds and slip manifolds allow access respectively from the outside of the mould to the drainage channel system and to the mould forming cavity filling and emptying system.
Functionally, the above-mentioned moulds may be considered on a level with a draining filter in which the ceramic mixture, cast in the forming cavity in the form of a water-based suspension of extremely fine solid particles, is held and moulded, whilst the liquid fraction separates from it through the surrounding forming surface which acts as a filter screen.
In practice, such moulds are controlled by a machine which controls the moulding cycle. At particular steps of the cycle, the mould drainage system may be supplied with the so-called service fluids (water, air and washing solutions). These may be supplied in two ways, that is to say, against the current or by absorption. When supplied against the current, the service fluids are introduced into the drainage system by means of the drainage manifolds, then flow down into the forming cavity, passing through the forming surfaces. During supply with absorption, with the flow parallel with the current, the service fluids are applied on the forming surfaces and left to migrate towards the drainage system by gravity or with the aid of a vacuum.
In the plumbing industry, the raw materials used for the ceramic mixtures, that is to say, the slips, are inorganic, obtained as a result of industrial refinement or directly from natural deposits. In the latter case, they may, therefore, contain impurities due to organic substances or other mineral compounds.
Slips normally consist of clays, feldspar and silica, finely ground and dispersed in water, of the industrial type. The solid particles in these ceramic mixtures have diameters measuring between several fractions of a μm up to around 40 μm.
Therefore, if, during use, a mould made of porous material (for example, a mould made of microporous resin) is not subjected to targeted and regular maintenance treatments, the pores may be partially or completely blocked, due to the natural penetration of particles from the ceramic mixture, or the infiltration of impurities from the air and/or water used for mould operation. Moreover, the filter layer of a mould may also accidentally be damaged by contamination by substances from outside the production cycle, such as greases, oils, etc.
The effects of the substances infiltrating the pores of the mould filter screen may be classed as: biological and organic contamination; inorganic encrustations; and mixed encrustations, which combine the various types indicated above.
In the case of biological contamination, the contaminating agents are the impurities contained in the mixtures or in the mould service water, such as humus and bacterial loads in general.
As indicated, organic contamination is due to the accidental presence of greases and/or oils.
Contamination by encrustations is due to the formation of clusters as a result of the interaction of particles in the mixture with salts or oxides. The latter may be present as impurities in the raw materials and/or in the water used to prepare the mixture, or may be introduced into the mould during the various steps of the technological cycle (for example, with water during mould washing).
Document DE-2 107 018 discloses a method for moulding ceramic products where compressed air is used to dry the porous moulds.
Document GB-1 337 492 discloses a method for moulding ceramic products and describes the use of warm air to speed up the process of drying the porous moulds.
Patent application EP-A-0 463 179 discloses a high-pressure device for moulding ceramic products in porous moulds and comprising an ultrasonic unit to clean the mould.
At present there are no known processes for regeneration of the mould materials which allow the full restoration of the original microporosity of the material. Therefore, there is no remedy to the progressive deterioration in the functionality of moulds made of resin, with consequent deterioration of production conditions in the specific steps of the process in question (object forming and removal from the mould). For these reasons, after a given period of use, the moulds must be substituted.